(1) Field of the Invention
The present invention relates to a method of producing alloy steels having an extremely low carbon content, and more particularly relates to a method of producing such high grade alloy steels having an extremely low carbon content inexpensively by promoting the decarburization reaction for alloy steel having a low carbon content with the use of a commonly used vacuum degassing apparatus without recourse to a special method.
(2) Description of the Prior Art
Steels having a very low carbon content are generally produced from molten steel by the vacuum degassing process which utilizes CO gas generated from the molten steel. As the vacuum degassing process, the RH circulating degassing process and VOD ladle degassing process have already been used in industry. However, in these vacuum degassing processes, CO gas pressure, which equilibrates with carbon and oxygen present in a molten steel having a very low carbon content, is close to the operation pressure of the vacuum apparatus, and CO bubbles are difficult to be generated, and hence the decarburization rate of the steel is very low. Therefore, it is considered that, when the carbon content of a molten steel is lower than a certain critical value, the decarburization of the steel no longer proceeds.
Moreover, when a molten steel containing chromium is decarburized, the following problems occur.
(1) The presence of chromium lowers the activities of carbon and oxygen in the molten steel, and hence the potential for forming CO gas of the steel is low.
(2) When it is intended to decrease the carbon content of the molten steel by increasing the oxygen content thereof based on equilibrium of carbon, oxygen and CO, chromium is always oxidized.
Therefore, it is very difficult to produce high-chrome steel having a very low carbon content.
In order to obviate these drawbacks, a stainless steel having an extremely low carbon content is commonly produced from a starting steel having an extremely low carbon content by a vacuum melting process or by an electron beam melting process. However, in both of these processes, the cost of the starting steel and the melting cost of the steel are very high, and mass production of the aimed stainless steel having an extremely low carbon content is impossible.